High-frequency devices are packaged into high-frequency electronic and optoelectronic components that provide convenient structures for handling, installing, and connecting the high-frequency devices to external circuits on an interconnection substrate, such as a printed circuit board. An interconnection substrate typically includes multiple layers of dielectric material (e.g., plastic or ceramic material) that support respective sets of electrically-conductive high-frequency signal traces, direct-current (DC) signal traces, and ground traces. Traces on different dielectric material layers typically are interconnected by electrically conducting vias that extend through the dielectric material layers. Both leadless and leaded high-frequency components may be electrically connected to the traces of an interconnection substrate using a variety of different automated, semi-automated, and manual component mounting processes.
In high-frequency circuits, a source is interconnected to a load (e.g., an integrated circuit chip) by a signal path that is modeled as a transmission line. In general, the source and the load are impedance-matched to the nominal impedance of the transmission line in order to minimize losses and reflections and achieve maximal power transfer from the source to the load. Any transition in the signal path (e.g., any change in the electrical or physical characteristics of the signal path) introduces discontinuities in the impedance of the signal path, causing signal reflections that degrade the integrity of the transmitted signal and that reduce the power transferred to the load. Signal vias, package leads, and bond wires are transitions that behave as parasitic inductances that cause significant reflections and significant degradation in the transmitted signal integrity, especially at frequencies in the GHz range and higher.
Different approaches have been proposed for compensating the parasitic inductances of the signal path transitions between a high-frequency component and the interconnection substrate. Many of these approaches are integrated into the designs of the interconnection interfaces of high-frequency component packages. For example, in one approach, high-frequency circuit elements are housed in a package in which the wires for leading out the electrodes of the circuit elements are formed of strip lines. An insulating film bonds the wires to a metal plate, which serves as a common ground for the strip lines and as a heat dissipation plate for the package. During mounting of the package to the interconnection substrate, the metal plate is bent and inserted into a slot within the interconnection substrate and soldered to a ground plane on the back side of the interconnection substrate; at the same time, the wires are bent and soldered to wiring layers on the top side of the interconnection substrate.
Many industry-standard component packages, such as transistor outline (TO) can packages and butterfly packages, do not include any type of integrated parasitic induction compensation of their external leads. These types of packages, however, frequently are used because of their relatively low cost and other considerations. At frequencies in the GHz range and higher, however, the signal degradation and electromagnetic interference resulting from the parasitic inductance discontinuities of the external leads significantly reduce the utility of these types of packages.
Several attempts have been made to overcome the difficulties associated with operating components with uncompensated external signal leads at high frequencies. In one approach, a metal shield is placed around the external leads to reduce electromagnetic interference (EMI) that is generated by passing high-frequency signals through the uncompensated external signal leads. This approach, however, does not address the need to reduce the losses and distortions of the high-frequency signals at the uncompensated signal leads. In another approach, a flexible circuit that includes a patterned copper metallization over a thin polyimide film is soldered between the leads of TO can package and the wiring traces of an interconnection substrate. The flexible circuit forms a coplanar wave guide between the TO can leads and the wiring traces of the interconnection substrate, thereby significantly reducing losses and reflections in the external leads. This approach, however, is characterized by high manufacturing costs, high defect rates, and poor reliability.